In recent years, attention has been paid to fuel cells as electric power sources for electric vehicles and stationary electric sources in concert with social requirements and movements on the background of energy and environmental problems. Fuel cells are classified into a variety of types according to kinds of electrolyte and kinds of electrode. Typical examples are alkaline type, phosphoric add type, molten carbonate type, solid electrolyte type, and polymer or solid polymer type. Of these, the spotlight of attention is focused on the polymer electrolyte fuel cell (PEFC) which is able to be operated at low temperatures (usually not higher than 100° C.) and which is in recent years developed for practical use as a low environmental pollution power source for automotive vehicle.
In general, PEFC includes an membrane-electrode assembly (MEA) sandwiched between separators. MEA in general has a laminate structure including a cathode gas diffusion layer (GDL), a cathode catalyst layer, a polymer or solid polymer electrolyte layer, an anode catalyst layer and an anode gas diffusion layer.
In MEA, the following electrochemical reactions proceed. First, hydrogen contained in fuel gas supplied to an anode (fuel electrode) side is oxidized to form protons and electrons by catalyst. Subsequently, the produced protons pass through a polymer electrolyte contained in the anode side catalyst layer and the polymer electrolyte membrane contacting with the anode side catalyst layer, and reaches the cathode (air electrode) side catalyst layer. The electrons produced in the anode side catalyst layer pass through an electrically conductive carrier constituting the anode side catalyst layer, the gas diffusion layer contacting to the anode side catalyst layer on the side opposite to the polymer electrolyte membrane, the separator and an outside circuit, and reach the cathode side catalyst layer. The protons and electrons reaching the cathode side catalyst layer react with oxygen contained in oxidizer gas supplied to the cathode side catalyst layer, and thereby produce water. In the fuel cell, it is possible to take out electricity to the outside through the above-mentioned electrochemical reactions.
In PEFC, water is required to retain the proton conductivity of the polymer electrode membrane, and insufficiency of water causes a condition in which PEFC becomes unable to continue the generation of electricity. This phenomenon is called dry-out. On the other hand, water is produced in the cathode, as mentioned before, and the produced water stays in the catalyst layer, GDL, and separator, and makes it difficult for oxygen to diffuse to the cathode catalyst layer, resulting in incapability of continuing the generation of electricity. This phenomenon is called flooding. As conceivable measures for improving the resistance to dry-out, it is possible to employ an electrolyte membrane capable of returning the water produced in the cathode to the anode quickly, or to decrease the drainage of water from MEA. As conceivable measures for improving the resistance to flooding, it is possible to employ the electrolyte membrane capable of returning the water produced in the cathode to the anode quickly, or to increase the drainage of water from MEA. However, the latter technique (the control of water drainage) for improving the dry-out resistance and the flooding resistance is difficult to achieve both objectives simultaneously since both are in a relationship of trade off.
For this problem, there is a report of a technique of providing a water retaining layer having an enhanced water holding property, between the electrode catalyst layer and the gas diffusion layer (Patent Document 1).
MEA disclosed in Patent Document 1 can improve the dry-out resistance with the water retaining layer. However, the flowing resistance remains poor under the humid condition, and therefore, the fuel cell using MEA disclosed in Patent Document 1 is insufficient in power generating performance.
Therefore, the present invention is devised in view of the above-mentioned situation, and aimed to provide an electrolyte membrane-electrode assembly capable of coping with both of the dryout resistance and the flooding resistance.